Adding memory to an old Macbook

I have a 2006 Macbook, and each memory slot has 512 MB. The machine has been pretty slow browsing the web lately, and I thought I'd try upgrading the memory. The machine will take a maximum of 2GB; I know memory is cheap, but I have a single 1GB memory card from an old Macbook Pro that I'm not using.
Is there any reason not to leave in one 512 MB card and replace the other one with the 1GB card? Do the two cards need to be the same size? Thanks.

The 512mb you have already is likely 2x256mb modules if it's the stock ram that came with the macbook so you have to remove 256 to put anything else in. Then you need to be sure that the 1gb module you have is the appropriate speed for you're Mac.

I'd suggest running the system scanner at Crucial.com to see what you have and whether it matches the 1gb module you have
Memory upgrades from Crucial.com - Crucial Mac System Scanner

It's possible, but not very likely, that the memory modules are compatible. Unless they are identical in nature, I'd say you're asking for trouble trying to cheap out on this, particularly given the low prices of MacBook RAM from that era.